Broadband satellite communication system using optical feeder links

ABSTRACT

Broadband satellite communications systems using optical feeder links are disclosed. Various optical modulation schemes are disclosed that can provide improved capacity for fixed spot beam, on board beamforming, and ground-based beamforming broadband satellite systems.

This application is a continuation of application no. PCT/US16/69628,filed 30 Dec. 2016, which claims the benefit of application No.62/273,730, filed 31 Dec. 2015, each of which is incorporated byreference herein.

TECHNICAL FIELD

The disclosed techniques relates to broadband satellite communicationslinks and more specifically to satellites using optical links forbroadband communication between satellite access nodes and thesatellites.

BACKGROUND

Satellite communications systems provide a means by which data,including audio, video and various other sorts of data can becommunicated from one location to another. The use of such satellitecommunications systems has gained in popularity as the need forbroadband communications has grown. Accordingly, the need for greatercapacity over each satellite is growing.

In satellite systems, information originates at a station (which in someinstances is a land-based, but which may be airborne, seaborne, etc.)referred to here as a Satellite Access Node (SAN) and is transmitted upto a satellite. In some embodiments, the satellite is a geostationarysatellite. Geostationary satellites have orbits that are synchronized tothe rotation of the Earth, keeping the satellite essentially stationarywith respect to the Earth. Alternatively, the satellite is in an orbitabout the Earth that causes the footprint of the satellite to move overthe surface of the Earth as the satellite traverses its orbital path.

Information received by the satellite is retransmitted to a user beamcoverage area on Earth where it is received by a second station (such asa user terminal). The communication can either be uni-directional (e.g.,from the SAN to the user terminal), or bi-directional (i.e., originatingin both the SAN and the user terminal and traversing the path throughthe satellite to the other). By providing a relatively large number ofSANs and spot beams and establishing a frequency re-use plan that allowsa satellite to communicate on the same frequency with several differentSANs, it may be possible to increase the capacity of the system. Userspot beams are antenna patterns that direct signals to a particular usercoverage area (e.g., a multi beam antenna in which multiple feedsilluminate a common reflector, wherein each feed produces a differentspot beam). However, each SAN is expensive to build and to maintain.Therefore, finding techniques that can provide high capacity with fewsuch SANs is desirable.

Furthermore, as the capacity of a satellite communication systemincreases, a variety of problems are encountered. For example, whilespot beams can allow for increased frequency reuse (and thus increasedcapacity), spot beams may not provide a good match to the actual needfor capacity, with some spot beams being oversubscribed and other spotbeams being undersubscribed. Increased capacity also tends to result inincreased need for feeder link bandwidth. However, bandwidth allocatedto feeder links may reduce bandwidth available for user links.Accordingly, improved techniques for providing high capacity broadbandsatellite systems are desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed techniques, in accordance with one or more variousembodiments, is described with reference to the following figures. Thedrawings are provided for purposes of illustration only and merelydepict examples of some embodiments of the disclosed techniques. Thesedrawings are provided to facilitate the reader's understanding of thedisclosed techniques. They should not be considered to limit thebreadth, scope, or applicability of the claimed invention. It should benoted that for clarity and ease of illustration these drawings are notnecessarily made to scale.

FIG. 1 is an illustration of an example of a satellite communicationssystem using radio frequency signals to communicate with the satelliteand having a relatively large number of satellite access nodes (“SANs”,also known as “gateways”) to create a high capacity system.

FIG. 2 is an illustration of a simplified satellite that uses RF signalsto communicate with SANs.

FIG. 3 is a simplified illustration of an example of the repeaters usedon the forward link.

FIG. 4 is a simplified schematic of an example of a first of the threesystem architectures in which an optical link is used to communicationon the feeder link.

FIG. 5 shows an example of the relationship of IF signals, opticalchannels and optical bands used by the system in some embodiments.

FIG. 6 shows an example of an optical transmitter used to perform theoptical modulation of the binary data stream onto the optical signals.

FIG. 7 is an illustration of an example of the return path for thesystem of FIG. 4.

FIG. 8 is a simplified schematic of an example of a third systemarchitecture in which an optical link is used to communicate on thefeeder link.

FIG. 9 is an illustration of an example of the relationship betweensub-channels, carriers and optical signals within the system of FIG. 8.

FIG. 10 is a simplified illustration of an example of a SAN.

FIG. 11 is an illustration of an example of the return link for thesystem of FIG. 8.

FIG. 12 is a simplified schematic of an example of a system architecturein which a satellite has on-board beamforming.

FIG. 13 is a simplified block diagram of an example of a weight/combinermodule.

FIG. 14 is a simplified schematic of an example of a system architecturein which an optical signal is RF modulated at a SAN and sent to asatellite that has on-board beamforming capability.

FIG. 15 is an illustration of an example of a forward link of asatellite communications system using ground-based beamforming andincluding an optical forward uplink and a radio frequency forwarddownlink.

FIG. 16 is an example of a forward beamformer used in a systemperforming ground-based beamforming.

FIG. 17 is a more detailed illustration of an example of the return linkcomponents within the example FIG. 18 is a simplified illustration ofcomponents of a satellite used for receiving and transmitting theforward link of an example system using ground-based beamforming.

FIG. 18 shows of an example of the components of a satellite in greaterdetail.

FIG. 19 is an illustration of an example of user beam coverage areasformed over the continental United States.

FIG. 20 is an illustration of an example of an optical transmitterhaving a timing module for adjusting the timing of the beam elementsignals and the timing pilot signal.

FIG. 21 is a system in which each of the K forward beam input signalscontain S 500 MHz wide sub-channels.

FIG. 22 is a simplified block diagram of an example of a beamformer.

FIG. 23 is an illustration of an example of a SAN.

FIG. 24 is an illustration of an example of a return link for a systemhaving ground-based beamforming.

FIG. 25 is an illustration of an example of one of the SANs in thereturn link.

FIG. 26 is an example of an illustration of an example return beamformer

The figures are not intended to be exhaustive or to limit the claimedinvention to the precise form disclosed. It should be understood thatthe disclosed techniques can be practiced with modification andalteration, and that the invention should be limited only by the claimsand the equivalents thereof.

DETAILED DESCRIPTION

Initially, a system that uses radio frequency (RF) communication linksbetween satellite access nodes (SANs) and a satellite is discussed.Following this introduction is a discussion of several opticaltransmission techniques for broadband capacity satellites. Following anintroductory discussion of systems having an optical feeder link, threetechniques are discussed for modulating signals on an optical feederlink. In addition, three architectures are provided for implementing thetechniques.

FIG. 1 is an illustration of a satellite communications system 100 inwhich a relatively large number of stations (referred to herein as“SANs”, also referred to as “gateways”) 102 communicate with a satellite104 using RF signals on both feeder and user links to create arelatively large capacity system 100. Information is transmitted fromthe SANs 102 over the satellite 104 to a user beam coverage area inwhich a plurality of user terminals 106 may reside. In some embodiments,the system 100 includes thousands of user terminals 106. In some suchembodiments, each of the SANs 102 is capable of establishing a feederuplink 108 to the satellite 104 and receiving a feeder downlink 110 fromthe satellite 104. In some embodiments, feeder uplinks 108 from the SAN102 to the satellite 104 have a bandwidth of 3.5 GHz. In someembodiments, the feeder uplink signal can be modulated using 16quadrature amplitude modulation (QAM). Use of 16 QAM modulation yieldsabout 3 bits per second per Hertz. By using 3.5 GHz bandwidth per spotbeam, each spot beam can provide about 10-12 Gbps of capacity. By using88 SANs, each capable of transmitting a 3.5 GHz bandwidth signal, thesystem has approximately a 308 GHz bandwidth or a capacity of about 1000Gbps (i.e., 1 Tbps).

FIG. 2 is an illustration of a simplified satellite that can be used inthe system of FIG. 1, wherein the satellite uses RF signals tocommunicate with SANs. FIG. 3 is a simplified illustration of therepeaters 201 used on the forward link (i.e., receiving the RF feederuplink and transmitting the RF user downlink) in the satellite of FIG.2. A feed 202 within the feeder link antenna (not shown) of thesatellite 104 receives an RF signal from a SAN 102. Although not shownin detail, the user link antenna can be any of: one or more multi beamantenna array (e.g., multiple feeds illuminate a shared reflector),direct radiating feeds, or other suitable configurations. Moreover, userand feeder link antennas can share feeds (e.g., using dual-band combinedtransmit, receive), reflectors, or both. In one embodiment, the feed 202can receive signals on two orthogonal polarizations (i.e., right-handcircular polarization (RHCP) and left-hand circular polarization (LHCP)or alternatively, horizontal and vertical polarizations). In one suchembodiment, the output 203 from one polarization (e.g., the RHCP) isprovided to a first repeater 201. The output is coupled to the input ofa Low noise amplifier (LNA) 304 (see FIG. 3). The output of the LNA 304is coupled to the input of a diplexer 306. The diplexer splits thesignal into a first output signal 308 and second output signal 310. Thefirst output signal 308 is at a first RF frequency. The second outputsignal 310 is at a second RF frequency. Each of the output signals 308,310 are coupled to a frequency converter 312, 314. A local oscillator(LO) 315 is also coupled to each of the frequency converters 312, 314.The frequency converters shift the frequency of the output signals to auser downlink transmission frequency. In some embodiments, the same LOfrequency is applied to both frequency converters 312, 314. The outputof the frequency converters 312, 314 is coupled through a channel filter316, 318 to a hybrid 320. The hybrid 320 combines the output of the twochannel filters 316, 318 and couples the combined signal to alinearizing channel amplifier 322.

Combining the signals within the hybrid 320 allows the signals to beamplified by one traveling wave tube amplifier (TWTA) 324. The output ofthe linearizing channel amplifier 322 is coupled to the TWTA 324. TheTWTA 324 amplifies the signal and couples the amplified output to theinput of a high pass filter and diplexer 326. The high pass filter anddiplexer 326 split the signal back into two outputs based on thefrequency of the signals, with a higher frequency portion of the signalbeing coupled to a first antenna feed 328 and a lower frequency portionof the signal being coupled to a second antenna feed 330. The firstantenna feed 328 transmits a user downlink beam to a first user beamcoverage area U1. The second antenna feed 330 transmits a user downlinkbeam to a second user beam coverage area U3.

The output 331 of the feed 202 from the second polarization (e.g., LHCP)is coupled to a second arm 332 of the repeater. The second arm 332functions in a manner similar to the first 201, however the outputfrequencies transmitted to the user beam coverage areas U2 and U4 willbe different from the frequencies transmitted to the user beam coverageareas U1 and U3.

In some embodiments, an optical link can be used to increase thebandwidth of the feeder uplink 108 from each SAN 102 to the satellite104 and the feeder downlink 110 from the satellite to each SAN 102. Thiscan provide numerous benefits, including making more spectrum availablefor the user links. Furthermore, by increasing the bandwidth of thefeeder links 108, 110, the number of SANs 102 can be reduced. Reducingthe number of SANs 102 by increasing the bandwidth of each feeder linkto/from each SAN 102 reduces the overall cost of the system withoutreducing the system capacity. However, one of the challenges associatedwith the use of optical transmission signals is that optical signals aresubject to attenuation when passing through the atmosphere. Inparticular, if the sky is not clear along the path from the satellite tothe SANs, the optical signal will experience significant propagationloss due to the attenuation of the signals.

In addition to attenuation due to reduced visibility, scintillationoccurs under adverse atmospheric conditions. Therefore, techniques canbe used to mitigate against the effects of fading of the optical signaldue to atmospheric conditions. In particular, as will be discussed ingreater detail below, the lenses on board the satellite used to receivethe optical signals and the lasers on board the satellite used totransmit optical signals can be directed to one of several SANs. TheSANs are dispersed over the Earth so that they tend to experience pooratmospheric conditions at different times (i.e., when fading is likelyon the path between the satellite and a particular SAN, it will berelatively unlikely on the path between the satellite and each of theother SANs).

By taking into account the differences in atmospheric conditions indifferent parts of the country, the decision can be made when theatmosphere between the satellite and a particular SAN is unfavorable tothe transmission of an optical signal, to use a different SAN to whichthe atmospheric conditions are more favorable. For example, thesouthwest of the continental United States has relatively clear skies.Accordingly, SANs can be located in these clear locations in the countryto provide a portal for data that would otherwise be sent through SANsin other parts of the country when the sky between those SANs and thesatellite is obstructed.

In addition to directing the satellite to communicate with those SANsthat have a favorable atmospheric path to/from the satellite, signalsthat are received/transmitted by the satellite through one of severaloptical receivers/transmitters can be directed to one of severalantennas for transmission to a selected user beam coverage area. Thecombination of flexibility in determining the source from which opticalsignals can be received on the optical uplink and the ability to selectthe particular antenna through which signals received from the sourcewill be transmitted allows the system to mitigate the negative impact ofthe variable atmospheric conditions between the SANs and the satellite.

As disclosed herein, at least three different techniques that can beused to communicate information from SANs through a satellite to userbeam coverage areas in which user terminals may reside. Three suchtechniques will now be described. A very brief summary of each isprovided, followed by a more detailed disclosure of each architecture.

Briefly, the first technique uses a binary modulated optical signal onthe uplink Several SANs each receive information to be transmitted touser terminals that reside within user beam coverage areas. The opticalsignal is modulated with digital information. In some embodiments, eachSAN transmits such a binary modulated optical signal to the satellite.The digital information may be a representation of information intendedto be transmitted to a user beam coverage area in which user terminalsmay reside. The signal is detected in the satellite using an opticaldetector, such as a photodiode. In some embodiments, the resultingdigital signal is then used to provide binary encoding, such as binaryphase shift keying (BPSK) modulate an intermediate frequency (IF)signal. The IF signal is then upconverted to a satellite RF downlinkcarrier frequency. Modulating the RF signal with BPSK can be donerelatively simply where the size, power, and thermal accommodation onthe satellite is small. However, using BPSK as the baseband modulationfor the RF signal on the user downlink 114 may not provide the maximumcapacity of the system. That is, the full potential of the RF userdownlink 114 is reduced from what it may be possible if a densermodulation scheme is used, such as 16 QAM instead of BPSK on the RF userdownlink 114.

The second technique also modulates the optical signal on the uplinkusing a binary modulation scheme. The modulated optical signal isdetected by a photodiode. The resulting digital signal is coupled to amodem. The modem encodes the digital information onto an IF signal usinga relatively bandwidth efficient modulation scheme, such as quadratureamplitude modulation (QAM). QAM is used herein to refer to modulationformats than encode more than 2 bits per symbol, including for examplequadrature phase shift keying (QPSK), offset QPSK, 8-ary phase shiftkeying, 16-ary QAM, 32-ary QAM, amplitude phase shift keying (APSK), andrelated modulation formats. While the use of the denser QAM schemeprovides a more efficient use of the RF user link, using such encodingon the RF user downlink 114 requires a relatively complexdigital/intermediate frequency (IF) conversion block (e.g., modem). Suchcomplexity increases the size, mass, cost, power consumption and heat tobe dissipated.

The third technique uses an RF modulated optical signal (as opposed tothe binary modulated optical signals of the first two techniques). Inthis embodiment, rather than modulating the optical signal with digitalinformation to be transmitted to the user beam coverage area, an RFsignal is directly modulated (i.e., intensity modulated) on to theoptical carrier. The satellite then need only detect the RF modulatedsignal from the optical signal (i.e., detect the intensity envelope ofthe optical signal) and frequency upconvert that signal to the userdownlink frequency, relieving the satellite of the need for a complexmodem. The use of an RF modulated optical signal increases the overallcapacity of the communications system by allowing a denser modulation ofthe user link RF signal, while reducing the complexity of the satellite.Due to the available bandwidth in the optical signal, many RF carrierscan be multiplexed onto an optical carrier. However, optical signalsthat are intensity modulated with an RF signal are susceptible to errorsdue to several factors, including fading of the optical signal.

Each of these three techniques suffer from the fact that there is anunreliable optical channel from the SANs to the satellite. Therefore,three system architectures are discussed to mitigate the problems ofunreliable optical feeder link channels. In each configuration,additional SANs are used to offset the inherent unreliability of theoptical links to the satellite. Signals can be routed from any of theSANs to any of the user beam coverage areas. Using additional SANsensures that a desired number of SANs that have a high quality opticallink to the satellite are available. Furthermore, flexibility in therouting through the satellite (i.e., referred to herein as “feeder linkdiversity”) allows data to be transmitted from those SANs that have thedesired quality optical channel to the satellite on the feeder link andto user spot beams on the user link in a flexible way.

Each of these three techniques will now be discussed in detail. Each ofthese techniques are discussed in the context of embodiments that have aparticular number of components (i.e., SANs, lasers per SAN,transponders within the satellite, etc.). However, such specificembodiments are provided merely for clarity and ease of the discussion.Furthermore, a wide range of IF and/or RF frequencies, opticalwavelengths, numbers of SANs, numbers of transponders on the satellite,etc. are within the scope of the disclosed embodiments. Therefore, theparticular frequencies, wavelengths, antenna array elements, and numbersof similar parallel channels, components, devices, user beam coverageareas, etc. should not be taken as a limitation on the manner in whichthe disclosed systems can be implemented, except where expressly limitedby the claims appended hereto.

FIG. 4 is a simplified schematic of a first of the three techniquesnoted above. A system 600 for implementing the first technique includesa plurality of SANs 602, a satellite 604 with at least one single-feedper beam antenna 638, 640 and a plurality of user terminals 606 withinuser beam coverage areas 1801 (see FIG. 19). Alternatively, any antennacan be used in which the antenna has multiple inputs, each of which canreceive a signal that can be transmitted in a user spot beam to a userbeam coverage area, such as direct radiating antennas, etc. The antennas638, 640 may be a direct-radiating array or part of a reflector/antennasystem. In some embodiments, the system 600 has M SANs 602. In theexample system 600 and for each of the example systems discussedthroughout this disclosure, M=8. However, none of the systems disclosedhere should be limited to this number. M=8 is merely a convenientexample, and in other embodiments, M can be equal to 2, 4, 10, 12, 16,20, 32, 40, or any other suitable value. In some embodiments, the SANs602 receive “forward traffic” to be communicated through the system froma source (such as a core node, not shown), which may receive informationfrom an information network (e.g., the Internet). The data communicatedto a SAN 602 from the core node can be provided in any form that allowsfor efficient communication of the data to the SAN 602, including as abinary data stream. In some embodiments, data is provided as a binarydata stream modulated on an optical signal and transmitted to the SAN onan optical fiber. Forward traffic is received in streams that areidentified with a particular user beam coverage area 1801. In someembodiments, the data may also be associated with a particular userterminal or group of user terminals to which the data is to betransmitted. In some embodiments, the data is associated with a terminalbased on the frequency and/or timing of the signal that carriers thedata. Alternatively, a data header or other identifier may be providedwith the data or included in the data or in the data.

Once received, the forward traffic is a binary data stream 601. That is,in some embodiments, the forward traffic is a binary representation,such as an intensity modulated or phase modulated optical signal. Inalternative embodiments, the forward traffic can be decoded into anyother binary representation.

FIG. 5 shows the relationship of IF signals 903, optical channels 915and optical bands 907, 909, 911, 913 used by the system in someembodiments. The particular selection of bandwidths, frequencies,quantities of channels and wavelengths are merely examples provided tomake disclosure of the concepts easier. Alternative modulation schemescan be used, as well as other optical wavelengths, quantities ofchannels and other RF and/or IF bandwidths and frequencies. The schemeshown is merely provided to illustrate one particular scheme that mightbe used. As shown, a plurality of 3.5 GHz wide binary modulated IFsignals (e.g. 64) 903 carry binary data to be transmitted in one userspot beam. Examples of other bandwidths that can be used include 500MHz, 900 MHz, 1.4 GHz, 1.5 GHz, 1.9 GHz, 2.4 GHz, or any other suitablebandwidth.

The binary (i.e., digital) content modulated onto each 3.5 GHz widebinary modulated IF signal 903 is used to perform binary intensitymodulation of one of 16 optical channels within one of 4 optical bands905. In some embodiments, the four bands 907, 909, 911, 913 of theoptical spectrum are 1100 nm, 1300 nm, 1550 nm and 2100 nm. However,bands may be selected that lie anywhere in the useful optical spectrum(i.e., that portion of the optical spectrum that is available at leastminimally without excessive attenuation through the atmosphere). Ingeneral, optical bands are selected that have no more attenuation thanbands that are not selected. That is, several optical bands may haveless attenuation then the rest. In such embodiments, a subset of thoseoptical bands are selected. Several of those selected bands may exhibitvery similar attenuation.

In one example, each optical channel is defined by the wavelength at thecenter of the channel and each optical channel is spaced approximately0.8 nm apart (i.e., 100 GHz wide). While the RF signal 903 that ismodulated onto the optical channel is only 3.5 GHz wide, the spacingallows the optical signals to be efficiently demultiplexed. In someembodiments, each SAN 602 wavelength division multiplexes (WDM) several(e.g., 64) such 3.5 GHz optical signals 903 (i.e., 4×16) together ontoan optical output signal. Accordingly, the digital content of 64 opticalchannels can be sent from one SAN 602.

FIG. 6 shows an optical transmitter 607 used to perform the opticalmodulation of the binary data stream 601 onto the optical signals. Inaccordance with the embodiment that implements the scheme shown in FIG.5, the optical transmitter 607 includes four optical band modules 608a-608 d (two shown for simplicity) and an optical combiner 609. Each ofthe 4 optical band modules 608 include 16 optical modulators 611 (twoshown for simplicity) for a total of 64 modulators 611. Each of the 64modulators 611 output an optical signal that resides in one of 64optical channels 915 (see FIG. 5). The channels are divided into 4optical bands 907, 909, 911, 913.

The modulator 611 determines the optical channel 915 based on thewavelength 1 of a light source 654 that produces an optical signal. AnMZM 652 intensity modulates the output of the first light source 654with an intensity proportional to the amplitude of the binary datastream 601. The binary data stream 601 is summed with a DC bias in asummer 656. Since the binary data stream 610 is a digital signal (i.e.,having only two amplitudes), the resulting optical signal is a binarymodulated optical signal. The modulated optical output from the MZMmodulator 652 is coupled to an optical combiner 609. For a system usinga modulation scheme such as the one illustrated in FIG. 5, each of the16 light sources 654 that reside within the same optical band module 608output an optical signal at one of 16 different wavelengths λ1. The 16wavelengths correspond to the 16 optical channels 915 within the firstoptical band 907. Likewise, the light sources 654 in the opticalmodulators 611 in each other optical band module 608 output an opticalsignal having a wavelength of λ1 equal to the wavelength of the channelsin the corresponding optical band 909, 911, 913. Accordingly, the 64optical outputs 915 from the four optical band modules 608 a-608 d eachhave a different wavelength and fall within the 16 optical channels ofthe four bands that are defined by the wavelengths 1 of signalsgenerated by the 64 light source 654. The optical combiner 609 outputs awavelength division multiplexed (WDM) optical signal 660 that is thecomposite of each signal 915.

The SAN 602 sends the optical signal 660 to the satellite 604 over anoptical feeder uplink 108 (see FIG. 4). The optical signal emitted bythe optical transmitter 607 is received by a lens 610 in the satellite604. In some embodiments, a lens 610 is part of a telescope within theoptical receiver 622. In some embodiments, the lens 610 is steerable(i.e., can be directed to point at any one of several SANs 602 withinthe system or any one from within a subset). By allowing the lenses 610to be pointed to more than one of the SANs 602, the lens 610 can bepointed to a SAN 602 having an optical path to the satellite that is notcurrently subject to signal fading. The lens 610 may be pointed usingmechanical 2-axis positioning mechanisms. Pointing of the lens may beaccomplished by measuring the receive signal strength of a signaltransmitted over the optical channel and using the signal strength toidentify when the lens is pointed at a SAN with an optical link ofsufficient quality (i.e., above a desired quality threshold). Eitherground commands or on-board processing may provide directions to thelens positioning mechanisms to correctly point the lens 610 at thedesired SAN 602.

The optical receiver 622 further includes an optical demultiplexer 650,such as a filter or prism. The optical receiver 622 has a plurality ofoutputs, each output corresponding with an optical wavelength. As shownin FIG. 4, the optical receiver 622 has 64 outputs. However, as notedabove, the particular frequency, number of optical bands and wavelengthselection, and thus the number of outputs from the optical receiver 622,are provided herein merely as an example and are not intended to limitthe systems, such as system 600, to a particular number.

In some embodiments, each wavelength resides within one of the fouroptical bands 907, 909, 911, 913. Each optical wavelength is at thecenter of an optical channel. Optical channels within one band arespaced approximately 0.8 nm (i.e., 100 GHz) apart. Making the opticalchannels spacing wide makes it easier to provide an opticaldemultiplexer 650 that can demultiplex the optical signal to provideeach of the 64 optical channels on a separate output. In someembodiments, an additional lens 613 is provided to focus the output ofthe optical demultiplexer 650 into the input of an optical detector,such as a photodiode 612. The photodiode 612 generates an electricalsignal by detecting the intensity envelope of the optical signal 660presented at an optical input to the photo diode. In some embodiments inwhich the optical signal 660 was intensity modulated to one of twointensity levels, the first intensity level representing a logical “1”results in an electrical signal having a first amplitude which alsorepresents a logical “1”. A second intensity level representing alogical “0” results in an electrical signal an amplitude representing alogical “0”. Therefore, the electrical signal is placed in a first statewhen the intensity of the optical signal 660 is in a state representinga logical “1” and placed in a second state when the intensity of theoptical signal 660 is in a state representing a logical “0”.Accordingly, the optical receiver has a plurality of digital outputs615. The electrical signal output from the digital output 615 of thephotodiode 612 is coupled to a modulator 614, such as a bi-phasemodulator. In some embodiments, such as the embodiment of FIG. 4, an LNA617 is provided between the photo diode 612 and the bi-phase modulator614. The output of the bi-phase modulator 614 is a BPSK modulated IFsignal (i.e., analog signal) having two phases. The BPSK modulator 614outputs a signal having a first phase representing a logical “1” inresponse to the electrical input signal at the first amplitude (i.e., inthe first state). When the input to the modulator 614 has an amplituderepresenting a logical “0” (i.e., the second state), the phase of theoutput of the BPSK modulator 614 is shifted to a second phase differentfrom the first phase. The output of the modulator 614 is coupled to theinput of a switch matrix 616.

In the simplified schematic of FIG. 4, a second SAN 602, lens 610,optical receiver 622 and plurality of bi-phase modulators 614 (i.e., 64)are coupled to the switch matrix 616. While only two SANs 602 are shownin FIG. 4, it should be understood that the satellite may receiveoptical signals from several SANs 602 (e.g., 8).

In some embodiments, the switch matrix 616 shown in FIG. 4 has aplurality of (e.g., 64) inputs for each lens 610. That is, if thesatellite 604 has 8 lenses 610, the matrix switch 616 has 512 inputs,each coupled to one of the modulators 614. The switch matrix 616 allowssignals at the outputs of the switch matrix 616 to be selectivelycoupled to inputs of the switch matrix 616. In some embodiments, anyinput can be coupled to any output. However, in some embodiments, onlyone input can be coupled to any one output. Alternatively, the inputsand outputs are grouped together such that inputs can only be coupled tooutputs within the same group. Restricting the number of outputs towhich an input can be coupled reduces the complexity of the switchmatrix 616 at the expense of reduced flexibility in the system.

The outputs of the switch matrix 616 are each coupled to an upconverter626. The upconverter 626 upconverts the signal to the frequency of theuser downlink carrier. For example, in some embodiments, the signaloutput from the switch matrix 616 is a 3.5 GHz wide IF signal. The 3.5GHz wide IF signal is upconverted to an RF carrier having a 20 GHzcenter frequency. The output of each upconverter 626 is coupled to acorresponding power amplifier 630. The output of each amplifier 630 iscoupled to one of a plurality of antenna input, such as a inputs (e.g.,antenna feeds not shown) of one of the antennas 638, 640. Accordingly,each of the outputs of the switch matrix 616 is effectively coupled to acorresponding one of the antenna inputs. In some embodiments, each inputof each antenna 638, 640 transmits a user spot beam to one user beamcoverage area 1801 (see FIG. 19). The switch matrix 616 is capable ofselecting which input (i.e., bi-phase modulator 614) is coupled to whichoutput (i.e., upconverter 626). Accordingly, when (or before) the signalfrom one of the SANs 602 fades and errors become intolerable, the switchmatrix 616 can couple the input of the upconverter 626 (i.e., theassociated antenna feed) to a SAN 602 that is sending an optical signalthat is not experiencing significant fading. In some embodiments, theswitch matrix 616 allows the content that is provided to the antennainputs to be time division multiplexed so that content from a particularSAN can be distributed to more than one user spot beam (i.e., antennafeed).

That is, when each lens 610 is receiving a signal from the SAN 602 towhich it is pointing, each of the 64 outputs from the optical receiver622 associated with that Lens 610 will have a signal. In the embodimentin which each antenna input to the antennas 638, 640 transmits a userspot beam to a particular user coverage area 1801, all of the usercoverage areas 1801 will receive a signal (assuming the switch matrix616 is mapped to couple each input to one output). The switch matrix 616selects which analog output from the bi-phase modulator 614 is to becoupled to each antenna input (e.g., transmitted to each feed of thesingle-feed per beam antenna 638, 640) (i.e., in each user spot beam).However, when the optical signal from a particular SAN 602 fades, asignal is still provided to all of the antenna inputs to ensure that nouser coverage areas 1801 loses coverage. Time multiplexing the signalsfrom one SAN to more than 64 antenna inputs allows one SAN 602 toprovide signals to more than 64 user coverage areas 1801. While thetotal capacity of the system is reduced, the availability of the systemto provide each user coverage area with content is enhanced. This isbeneficial in a system with an optical feeder link. In some embodiments,such time multiplexing is done for a short time while the lens 610 thatis directed to a SAN 602 that has a weak optical link is redirected toanother SAN to which there is a stronger optical link More generally,the matrix 616 can be used to time multiplex analog signals output fromthe optical receiver 622 to more than one user spot beam, such thatduring a first period of time the analog signal is coupled to a firstantenna input (e.g., feed) transmitting a user spot beam directed to afirst user beam coverage area. During a second period of time, theanalog signal is coupled to a second antenna input (e.g., feed)transmitting a user spot beam directed to a second user beam coveragearea.

Once each lens 610 is receiving a sufficiently strong optical signal,the switch matrix 616 can again map each output to a unique output in aone-to-one correspondence of input to output. In some such embodiments,control of the switch matrix 616 is provided by a telemetry signal froma control station. In most embodiments, since all 64 of the IF signalsfrom the same SAN 602 will degrade together, the switch matrix 616 needonly be able to select between K/64 outputs, where K is the number ofuser spot beams and 64 is the number of photo diodes 612 in one opticalreceiver 622. As noted above, the process of controlling the routingthrough the satellite to map SANs 602 to user spot beams is referred toherein as feeder link diversity. As will be discussed below, feeder linkdiversity can be provided in three different ways.

In some embodiments, the satellite 604 has more antenna inputs thantransponders (i.e., paths from the optical receiver to the switches 634,636). That is, a limited number of transponders, which include poweramplifiers (PAs) 630, upconverters 626, etc., can be used to transmitsignals to a relatively larger number of user beam coverage areas. Bysharing transponders among antenna inputs, the output from each photodiode 612 can be time multiplexed to service a number of user beamcoverage areas that is greater than the number of transponders providedon the satellite 604. In this embodiment, RF switches 634 are used todirect the output of the PA 630 to different inputs of the one or bothof the antennas 638, 640 at different times. The times are coordinatedso that the information present on the signal is intended to betransmitted to the user beam coverage area to which the input isdirected (i.e., the feed is pointed). Accordingly, one transponder canbe used to provide information to several user beam coverage areas in atime multiplexed fashion. By setting the switches 634, 636 to direct thesignal to a particular antenna 638, 640, the signal received by each ofthe lenses 610 can be directed to a particular spot beam. This providesflexibly in dynamically allocating capacity of the system.

The switches 634, 636 direct the signal to inputs of any of the antennas638, 640 mounted on the satellite. In some embodiments, the output fromthe switches 634, 636 may be directed to a subset of the antennas. Eachantenna 638, 640 is a single-feed per beam antenna directed to aparticular user beam coverage area, thereby producing a spot beam. Inalternative embodiments, the PAs 630 may be directly connected to theantenna inputs, with the matrix switch 616 determining which of thesignals detected by each particular photo diodes 612 will be transmittedto which of the user beam coverage areas. In addition, even inembodiments in which there are an equal number of satellite transpondersand antenna inputs, having switches 634, 636 can reduce the complexityof the switch matrix 616. That is, using a combination of the switchmatrix 616 and switches 634, 636, the switch matrix 616 need not becapable of coupling each input to each output. Rather, the matrixinputs, outputs and antenna inputs can be grouped such that any input ofa group can be coupled only to any output of that same group. Theswitches 634, 636 can switch between antenna inputs (e.g., feeds) toallow outputs of one group to be coupled to an antenna input of anothergroup.

The switch matrix 616 may be operated statically or in a dynamic timedivision multiple access mode. In the static mode of operation, theconfiguration of the paths through the switch matrix 616 essentiallyremains set for relatively long periods of time. The configuration ofthe switch matrix 616 is only changed in order to accommodate relativelylong-term changes in the amount of traffic being transmitted, long termchanges in the quality of a particular link, etc. In contrast, in adynamic time division multiple access mode, the switch matrix 616 isused to time multiplex data between different forward downlink antennainputs. Accordingly, the switch matrix 616 selects which inputs tocouple to the output of the switch matrix 616. This selection is basedon whether the input signal is strong enough to ensure that the numberof errors encountered during demodulation of the signal at the userterminal 842, 844 is tolerable. In some such embodiments, timemultiplexing the analog outputs of the optical receiver 622 to differentantenna inputs allows one SAN 602 to service more than one user beamcoverage area. During a first period of time, one or more signals outputfrom an optical receiver 622 can each be coupled through to a unique oneof a first set of antenna inputs (i.e., directed to a unique one of afirst set of user beam coverage areas). During a second period of time,one or more of those same signals can be coupled through to differentantenna inputs (i.e., different user beam coverage areas). Such timemultiplexing of the analog outputs 615 from the optical receiver 622 canbe performed in response to one of the lens 610 of an optical receiver622 pointing to a “weak” SAN 602 (i.e., a SAN 602 having an optical linkthat is below a quality threshold). In such a embodiment, a first datastream initially set to the weak SAN 602 can be redirected by the corenode to a “strong” SAN 602 (i.e., a SAN 602 having an optical link thatis above the quality threshold). The strong SAN 602 time multiplexesthat information such that for a portion of the time the strong SAN 602transmits information directed to a first set of user beam coverageareas to which the first data stream is intended to be sent. During asecond period of time, the strong SAN 602 transmits a second data streamdirected to a second set of user beam coverage areas. Accordingly,during one period of time, information that would have been blocked fromreaching the satellite 604 by the poor optical link between the weak SAN602 and the satellite 604 can be transmitted to the satellite 604through the strong SAN 602. During this time, the lens 610 that ispointing at the weak SAN 602 can be redirected to point to a strong SAN602 that is not already transmitting to the satellite 604. As notedabove, this process of redirecting information from a weak SAN to astrong SAN is an aspect of feeder link diversity.

By determining when a feeder uplink signal is experiencing anunacceptable fade, data can be routed away from the SAN 602 that isusing the failing feeder uplink and to a SAN 602 that has a feederuplink signal that has an acceptable signal level. By the process offeeder link diversity, the signal transmitted through the selected SAN602 can then be routed through the switch matrix 616 to the spot beam towhich data is intended to be sent.

The system 600 has the advantage of being relatively simple to implementwithin the satellite 604. Conversion of binary modulated optical data toa BPSK modulated IF signal using photodiodes 612 and bi-phase modulators614 is relatively simple. Such bi-phase modulators are relatively easyand inexpensive to build, require relatively little power and can bemade relatively small and lightweight. However, using BPSK modulation onthe RF user downlink 114 is not the most efficient use of the limited RFspectrum. That is, greater capacity of the RF user downlink 114 (seeFIG. 1) can be attained by using a denser modulation scheme, such as 16QAM instead of BPSK on the RF user downlink 114.

For example, in an alternative embodiment of the system 600 thatimplements the second of the three techniques noted above, the analogsignal 618 that is to be transmitted on the user downlink is modulatedwith a denser modulation scheme. Generating the complex modulation onthe analog signal 618 requires that the modulator be a very complexmodulator that takes the digital data stream and converts the datastream to one or more complex modulated signals. The complex modulatedsignal 618 can be a high order modulation such as 64-QAM, 8 psk, QPSKfor example. Alternatively, any other modulation scheme can be used thatis capable of modulating symbols onto an IF carrier, where the symbolsrepresent more than two logical states. That is, the binary intensitymodulation of the optical signal results in the output 615 of theoptical receiver 622 providing an electronic signal that has binarymodulation representing the underlying content. In order to modulate theanalog signal 618 with a more complex modulation scheme, such as 16 QAM,the modulator 614 is a QAM modulator and thus perform QAM modulation ofthe IF signal based on the digital content output from the photodiode612.

Accordingly, in some embodiments, the bi-phase modulator 614 of thesystem 600 is replaced with a QAM modulator 614 (i.e., a modulator inwhich each symbol represents more than 2 bits). Accordingly, rather thanlimiting the modulation of the IF signals 618 to a binary modulationscheme (i.e., two logical states), such as BPSK, the modulator 614allows the IF signals 618 to be modulated with a denser modulationscheme (i.e., schemes in which symbols are capable of representing morethan two values, such as QAM). While the more complex QAM modulatorprovides a more efficient modulation of the IF signals 618 (QAM versesBPSK), it is more complex, requires more power, is heavier and moreexpensive than a bi-phase modulator.

FIG. 7 is an illustration of the return path for the system 600. Userterminals 606 transmit a binary modulated signal to the satellite 604.Switches 402 coupled to each element of the antenna (e.g., single beamper feed antennas 404, 406) select between satellite transponderscomprising a Low noise amplifier (LNA) 408, frequency converter 409 anddigital decoder 410. The frequency converter 409 down converts thereceived signal from the user uplink frequency to IF. The decoders 410decode the binary modulation on the received IF signal. Accordingly, theoutput of each decoder 410 is a digital signal. The digital decoders 410are coupled to inputs to a switch matrix 416. The switch matrix 416allows signals that are received over each of the user spot beams to bemodulated on different optical links (i e, transmitted to different SANs602) depending upon whether there is significant fading on the opticaldownlink to each SAN 602. The outputs of the switch matrix 416 arecoupled to inputs to optical transmitters 607. Each optical transmitter607 is essentially identical to the optical transmitter 607 shown inFIG. 6 and discussed above. In some embodiments in which the opticalspectrum is used in essentially the same manner as used on the forwardfeeder link (see FIG. 5), each of four optical band modules 608 receive16 outputs from the matrix switch 416 for a total of 64 inputs to theoptical transmitter 607. In some embodiments in which the satellite canreceive optical signals from 8 SANs 602, there are 8 such opticaltransmitters 607 that can receive a total of 512 outputs from the switchmatrix 416. Each optical transmitter 607 outputs an optical signal 660.The optical signal 660 is receive by a lens 412 within an opticalreceiver 414 in a SAN 602. The optical receiver 414 and lens 412 areessentially identical to the optical receiver 622 and lens 610 withinthe satellite 604, as described above with reference to FIG. 4.Accordingly, the output of the optical receiver 414 is a binary datastream. The output of the optical receiver is sent to an informationnetwork, such as the network that provided forward traffic to the SAN602.

In an alternative embodiment, the return link for the system 600, themodulation used on the return uplink from the user terminals 606 to thesatellite 604 is a more efficient modulation scheme than binarymodulation. Accordingly, the binary modulate 410 is a more complexmodulator 410. The binary data output from the demodulator 410 is theresult of decoding the modulated symbols modulated onto the IF signal bythe user terminal 606. For example, if 16 QAM was used on the useruplink, then the signal output from the demodulator is a digital streamof values represented by 16 QAM symbol. The binary signal output fromthe converter 502 is coupled to an input to the switch matrix 416. Boththe binary demodulator and the complex demodulator 410 output a digitaldata stream to be used to perform binary modulation of the opticalsignal transmitted on the feeder downlink by the optical transmitter607.

FIG. 8 is a simplified schematic of a system 800 for implementing thethird technique. In some embodiments of the system 800, a SAN 802receives the forward traffic as “baseband” signals 809 that are coupledto the inputs of a baseband to IF converter 1605. In some embodiments,seven 500 MHz wide baseband sub-channels 809 are combined in a 3.5 GHzwide IF signal 811. Each of the 3.5 GHz wide signals 811 is transmittedto one user coverage area 1801. FIG. 9 illustrates the relationshipbetween baseband sub-channels 809, IF signals 811 and optical signalswithin the system 800. Examples of other bandwidths that can be usedinclude 500 MHz (e.g., a single 500 MHz sub-channel), 900 MHz, 1.4 GHz,1.5 GHz, 1.9 GHz, 2.4 GHz, or any other suitable bandwidth.

FIG. 10 is a simplified illustration of a SAN 802, such as the SAN 802shown in FIG. 8. In some embodiments, there are 64 baseband to IFconverters 1605, shown organized in four IF combiners 1602, eachcomprising 16 converters 1605. Grouping of the baseband to IF converters1605 within IF converters 1602 is not shown in FIG. 8 for the sake ofsimplifying the figure. Each of the 64 baseband to IF converters 1605has S inputs, where S is the number of sub-channels 809. In someembodiments in which the sub-channel 809 has a bandwidth of 500 MHz andthe signal 811 has a bandwidth of 3.5 GHz, S equals 7. Each inputcouples one of the sub-channels 809 to a corresponding frequencyconverter 1606. The frequency converters 1606 provide a frequency offsetto allow a subset (e.g., S=7 in FIG. 10) of the sub-channels 809 to besummed in a summer 1608. Accordingly, in some embodiments, such as theone illustrated in FIG. 10, a SAN 802 processes 64 channels, each 3.5GHz wide. In some embodiments, the 3.5 GHz wide signal can be centeredat DC (i.e., using zero IF modulation). Alternatively, the signal 811can be centered at a particular RF frequency. In one particularembodiment, an RF carrier 811 is centered at the RF downlink frequency(in which case the satellite will need no upconverters 626, as describedfurther below). The output 811 from each summing circuit 1608 is an IFsignal 811 that is coupled to one of 64 optical modulators 611. The 64optical modulators 611 are grouped into 4 optical band modules 608. Eachoptical modulator 611 operates essentially the same as the opticalmodulator 611 shown in FIG. 6 and discussed above. However, since theinput 811 to each optical modulator 608 is an analog signal, the opticalsignal output from each optical modulator 611 is an intensity modulatedoptical signal having an amplitude envelope that follows the amplitudeof the IF signal 811.

An optical combiner 609 combines the outputs from each of the 64 opticalmodulators 611 to generate a wavelength division multiplexed (WDM)composite optical signal 1624. The number of baseband to IF converters1605 and the number of optical modulators 611 in the optical band module608 can vary. As shown in FIG. 9, the four optical modulators 611 can bedesigned to output optical signals having wavelengths centered at 1100nanometers, 1300 nanometers, 1550 nanometers and 2100 nanometers.

In the system 800, the optical transmitter 607 (similar to the opticaltransmitter 607 of FIG. 4) emits an RF modulated composite opticalsignal 1624. The RF modulated composite optical signal 1624 is receivedwithin the satellite 804 by a lens 610 (see FIG. 8). The lens 610 can bedirected to any of a plurality of SANs 802 capable of transmitting anoptical signal to the satellite 804. The output of the lens 610 iscoupled to the input of an optical detector, such as a photodiode 612(e.g. a PIN diode). The photodiode 612 detects the envelope (i.e., thecontour of the intensity) of the optical signal and converts theenvelope of the optical signal to an electrical signal. Since theoptical signal is intensity modulated with the IF signal 811, theresulting electrical signal output from the photodiode 612 isessentially the same as the IF signal 811 that was modulated by the SAN802 onto the composite optical signal 1624. The photodiode 612 iscoupled to an amplifier 808. The signal output from the amplifier 808 isthen coupled to an input of a matrix switch 616. The matrix switch 616performs in the same way as the matrix switch 616 discussed with respectto FIG. 4 above. Accordingly, the switch matrix 616 selects which inputsto couple to the output of the switch matrix 616. The output of thematrix switch 616 is handled the same as in the systems 600 describedabove in embodiments in which the signal 811 is at zero IF. Inembodiments in which the signal 811 output from the baseband to IFmodule 607 within the SAN is at a frequency that is to be directlytransmitted from the satellite 804, then the handling will be the same,but for the fact that the upconverters 626 are not required.

FIG. 11 is an illustration of the return link for the system 800. Thereturn link for the system 800 is essentially the same as shown in FIG.7. However, rather than the user terminals 606 transmitting a signalhaving binary modulation, the user terminals 606 transmit a signalhaving a more efficient modulation (e.g., 16 QAM rather than QPSK).Accordingly, the output digital decoder 410 is not required. Thedownconverter 850 downconverts the RF frequency used on the user uplinkto an appropriate IF frequency. In some embodiments, the IF frequencysignal is a zero IF signal that is 3.5 GHz wide. The output of eachdownconverter 850 is coupled to an input of the switch matrix 416.Therefore, the inputs of the MZM modulator 652 (see FIG. 6) receive ananalog signal from the switch matrix 416. Accordingly, the output ofeach optical modulator 611 is an intensity modulated optical signal inwhich the intensity envelope tracks the signal output from thedownconverter 850. In some embodiments, the optical modulator 611directly modulates the RF user uplink frequency onto the optical signal.Accordingly, the frequency converter 850 is not required. In embodimentsin which the downconverter 850 reduces the user uplink frequency to azero IF signal, the combined optical signal 660 is handled in the sameway as discussed with regard to FIG. 7. In embodiments in which theoptical signal is modulated with the user uplink frequency, adownconverter may be included within the modem 418 or prior to couplingthe signal from the optical receiver 414 to the modem 418.

Having discussed the three different techniques for modulating signalson the feeder link, each of which use a first system architecture havinga satellite that uses a matrix switch 616 to allow a flexible assignmentof received carriers to user spot beams, a second and third systemarchitectures are discussed. The second system architecture includes asatellite having on-board beam forming. The third system architectureuses ground-based beam forming.

FIG. 12 is a simplified schematic of a system 1000 using the techniqueshown in FIG. 4 (i.e., modulating the optical feeder uplink with binarymodulation and using that binary content to modulate an RF userdownlink) However, the system 1000 uses the second system architecturein which a satellite 1004 is capable of performing on-board beamforming.The system 1000 operates similarly to the system 600 described above.However, the IF output from each bi-phase modulator 614 is coupled to aweight/combiner module 1006 rather than to the switch matrix 616.

FIG. 13 is a simplified block diagram of a weight/combiner module 1006in which K forward beam signals 1002 are received in the weight/combinermodule 1006 by a beamformer input module 1052. The K signals 1002 arerouted by the input module 1052 to an N-way splitting module 1054. TheN-way splitting module 1054 splits each of the K signals 1002 into Ncopies of each forward beam signal, where N is the number of elements inthe antenna array that is to be used to form K user spot beams.

In the example of the system described above with respect to FIG. 4,there are 8 active SANs, each transmitting an optical signal comprising64 optical channels. Each of the 64 optical channels carries a 3.5 GHzIF signal (i.e., forward beam signal). Therefore, there are 512 forwardbeam signals (i.e., 8 SANs×64 IF signals). Accordingly K=512. In someembodiments, the satellite has an antenna array 1008 having 512 arrayelements. Accordingly, N=512.

Each output from the N-way splitting module 1054 is coupled to acorresponding input of one of 512 weighting and summing modules 1056.Each of the 512 weighting and summing modules 1056 comprises 512weighting circuits 1058. Each of the 512 weighting circuits 1058 place aweight (i.e., amplify and phase shift) upon a corresponding one of 512signals output from the N-way splitting module 1054. The weightedoutputs from the weighting circuits 1058 are summed by a summer 1060 toform 512 beam element signals 1062. Each of the 512 beam element signals1062 is output through a beamformer output module 1064. Looking back atFIG. 12, the 512 beam element signals 1062 output from theweight/combiner module 1006 are each coupled to a corresponding one of512 upconverters 626. The upconverters 626 are coupled to PAs 630. Theoutputs of the PAs 630 are each coupled to a corresponding one of 512antenna elements of the antenna array 1008. The antenna array can be anyof: a direct radiating array (where each antenna element directlyradiates in the desired direction), an array fed reflector (where eachantenna element illuminates a reflector shared by all antenna elements),or any other suitable antenna configuration. The combination of theantenna array 1008 and the weight combiner module 1006 is also referredto as a phased array antenna.

The relative weights of the signals being applied to the elements ateach of the locations within the phase array antenna 1008 will result inthe plurality of weighted signals superposing upon one another and thuscoherently combining to form a user beam.

Accordingly, by applying desired weighting to the plurality of signals1002 to generate the beam element signals 1062 output from theweight/combiner module 1006, a signal 1002 applied to each input of theweight/combiner module 1006 can be directed to one of the plurality ofuser beam coverage areas. Since the satellite 1004 can use theweight/combiner module 1006 and array antenna 1008 to direct any of thereceived signals to any of the user beam coverage areas, informationthat would otherwise be transmitted over a particular feeder uplink thatis experiencing intolerable fading can be routed to one of the otherSANs. Accordingly, the information can be transmitted to the satellite1004 through a SAN 602 that is not experiencing intolerable fading toprovide feed link diversity, as described above in the context of thematrix switch 616. Similar time division multiplexing can be done totransmit signals received by one of the lenses 610 in several user spotbeams as described above.

Using a satellite 1004 that has on-board beamforming providesflexibility to allow feeder link diversity with regard to signalsreceived from the plurality of SANs 602. The use of on-board beamforming eliminates the need for the switch matrix 616 shown in FIG. 4. Asimilar architecture can be employed on the return paths (i.e., the useruplink and the feeder downlink) That is, the user ground terminals 606transmit an RF signal up to the satellite 1004 on the user uplinkReceive elements in the antenna array 1008 receive the RF signal. Theweight/combiner module 1006 weights the received signals received byeach receive element of the antenna 1008 to create a receive beam. Theoutput from the weight/combiner module 1006 is down converted from RF toIF.

In some embodiments, the upconverters 626 are placed at the input of theweight/combiner module 1006, rather than at the outputs. Therefore, RFsignals (e.g., 20 GHz signals) are weighted and summed. The beam elementsignals are then transmitted through each of the antenna array elements.

In some embodiments, the satellite has several weight/combiner modules(not shown for simplicity). The inputs to each weight/combiner moduleare coupled to one or more optical receivers 622. In some embodiments,all of the outputs from one optical receiver 622 are coupled to the sameweight/combiner module. Each weight/combiner module generates N outputs.The N outputs from each weight/combiner module are coupled one-to-one toelements of one N-element antenna array (only one shown for simplicity).Accordingly, there is a one-to-one relationship between the antennaarrays 1008 and the weight/combiner modules 1006.

In some embodiments, the second architecture shown in FIG. 12 (i.e.,on-board beam forming) is used with a QAM modulator 614, similar to thesystem 600. However, the satellite 1104 has on-board beamforming.

FIG. 14 is a simplified schematic of a system 1200 using the techniquediscussed with respect to FIG. 8 in which an optical signal is RFmodulated at the SAN 802. However, the satellite architecture is similarto that of FIGS. 12 and 11 in which a satellite 1204 has on-boardbeamforming capability. The SANs 802, lenses 810, optical detectors(such as photodiodes 812), amplifiers 613 and upconverters 626 are allsimilar to those described with respect to FIG. 8. However, theweight/combiner module 1006 and array antenna 1008 are similar to thosedescribed with respect to FIGS. 10, 10A and 11. Similar to thearchitecture described in FIG. 12, the weight/combiner 1006 and arrayantenna 1008 allow the satellite 1004 to transmit the content of thesignals received from one or more of the SANs 802 to any of the userbeam coverage areas, thus providing feeder link diversity. Therefore, ifone or more of the feeder uplinks from the SANs 802 to the satellitehave an intolerable fade, the content that would otherwise be sent onthat feeder uplink can instead be sent through one of the other SANs 802using a feeder uplink that is not experiencing an intolerable fade.

FIG. 15 is an illustration of a forward link of a satellitecommunications system 1400 using the third system architecture (i.e.,ground-based beamforming) including an optical forward uplink 1402 and aradio frequency forward downlink 1404. In some embodiments, the system1400 includes a forward link ground-based beamformer 1406, a satellite1408 and a relatively large number (M) of SANs 1410 to create arelatively large capacity, high reliability system for communicatingwith user terminals 806 located within 512 user beam coverage areas 1801(see FIG. 19 discussed in detail below). Throughout the discussion ofthe system 1400, M=8 SANs 1410 are shown in the example. However, M=8 ismerely a convenient example and is not intended to limit the systemdisclosed, such as system 1400, to a particular number of SANs 1410.Similarly, 64 optical channels are shown in the example of the system1400. Likewise, the antenna array is shown as having 512 elements. Asnoted above, the particular frequencies, wavelengths, antenna arrayelements, and numbers of similar parallel channels, components, devices,user beam coverage areas, etc. should not be taken as a limitation onthe manner in which the disclosed systems can be implemented, exceptwhere expressly limited by the claims appended hereto.

Forward traffic (i.e., forward beam input signal 1407) to becommunicated through the system 1400 is initially provided to thebeamformer 1406 from a source, such as the Internet, throughdistribution equipment, such as a core node or similar entity (notshown). The distribution equipment may manage assignment of frequencyand/or time slots for transmissions to individual user terminals andgroup together data destined for transmission to particular beams, inaddition to performing other functions. Input signals 1407 to thebeamformer 1406 (or some portion of the information carried by theforward beam input signal 1407) can represent data streams (or modulateddata streams) directed to each of 512 user beams. In one embodiment,each of the 512 forward beam input signals 1407 is a 3.5 GHz wide IFsignal. In some embodiments, the forward beam input signal 1407 is acomposite 3.5 GHz wide carrier that is coupled to the input of thebeamformer 1406.

Each of the forward beam input signals 1407 is “directed” to a user beamcoverage area 1801 by the beamformer 1406. The beamformer 1406 directsthe forward beam input signal 1407 to a particular user beam coveragearea 1801 by applying beam weights to the 512 forward beam input signals1407 to form a set of N beam element signals 1409 (as further describedbelow with respect to FIG. 16). Generally, N is greater than or equal toK. In some embodiments, N=512 and K=512. The 512 beam element signals1409 are amplified and frequency converted to form RF beam elementsignals 1411. Each is transmitted from an element of an N-element (i.e.,512-element) antenna array 1416. The RF beam element signals 1411superpose on one another within the user beam coverage area 1801. Thesuperposition of the transmitted RF beam element signals 1411 form userbeams within the user beam coverage areas 1801.

In some embodiments, the 512 beam element signals 1409 are divided amongseveral SANs 1410. Accordingly, a subset of the beam element signals1409 (e.g., 512/8) are coupled to each SAN 1410, where 8 is the numberof SANs 1410. Thus, the combination of 8 SANs 1410 will transmit 512beam element signals 1409 from the beamformer 1406 to the satellite1408. In some embodiments, the beamformer 1406 is co-located with one ofthe SANs 1410. Alternatively, the beamformer 1406 is located at anothersite. Furthermore, in some embodiments, the beamformer 1406 may bedistributed among several sites. In one such embodiment, a portion ofthe beamformer 1406 is co-located with each SAN 1410. Each such portionof the beamformer 1406 receives all of the forward traffic 1407, butonly applies beam weights to those 64 (i.e., 512/8) signals 1409 to betransmitted to the SAN 1410 that is co-located with that portion of thebeamformer 1406. In some embodiments, several beamformers are provided(not shown for simplicity). Each beamformer generates N outputs (i.e.,beam element signals). The N beam element signals will be coupledone-to-one to elements of one N-element antenna array on the satellite1408 (only one shown for simplicity). Accordingly, there is a one-to-onerelationship between the antenna arrays 1416 and the beamformers 1406.In some embodiments in which all of the beam elements from onebeamformer 1406 are transmitted to the satellite 1408 through one SAN1410, there is no need to coordinate the timing of the transmissionsfrom different SANs 1410. Alternatively, in embodiments in which beamelements output from the same beamformer 1406 are transmitted to thesatellite 1408 through different SANs, the timing of the beam elementsignals is taken into consideration using timing controls as discussedfurther below.

The phase relationship between each of the RF beam element signals 1411transmitted from each of the N elements of an antenna array 1416 and therelative amplitude of each, determines whether the beam element signalswill be properly superpose to form beams within the desired user beamcoverage areas 1801. In some embodiments in which there are 8 SANs 1410(i.e., M=8) each SAN 1410 receives 64 beam element signals 1409.

In order to maintain the phase and amplitude relationship of each of the512 RF beam element signals 1411 to one another, the beamformer 1406outputs 8 timing pilot signals 1413, one to each SAN 1410, in additionto the N beam element signals 1409. Each timing pilot signal 1413 isaligned with the other timing pilot signals upon transmission from thebeamformer 1406 to each SAN 1410. In addition, the amplitude of eachtiming pilot signal 1413 is made equal.

FIG. 16 is a detailed illustration of the forward beamformer 1406. Theforward beamformer 1406 receives 512 forward beam signals 1407representing the forward traffic to be sent through the system 1400. Thesignals 1407 are received by a matrix multiplier 1501. The matrixmultiplier 1501 includes a beamformer input module 1502, a 512-waysplitting module 1504 and 512 weighting and summing modules 1506. Otherarrangements, implementations or configurations of a matrix multipliercan be used. Each of the 512 forward beam signals 1407 is intended to bereceived within a corresponding one of 512 user beam coverage areas1801. Accordingly, there is a one-to-one relationship between the 512user beam coverage areas 1801 and the 512 forward beam signals 1407. Insome embodiments, the distribution equipment (e.g., the core node) thatprovides the forward traffic to the beamformer 1406 ensures thatinformation to be transmitted to a particular user beam coverage area1801 is included within the forward beam input signal 1407 correspondingto that user beam coverage area 1801.

The 512-way splitting module 1504 splits each of the 512 forward beamsignals 1407 into 512 identical signals, resulting in 512×512 (i.e.,N×K) signals being output from the 512-way splitting module 1504. When Nis equal to 512 and K is equal to 512, the splitting module 1504 outputs512×512=524,288 signals. 512 unique signals output from the splittingmodule 1504 are coupled to each of the 512 weighting and summing modules1506. The signals coupled to each of the weighting and summing modules1506 are weighted (i.e., phase shifted and amplitude adjusted) inaccordance with beam weights calculated by a forward beam weightgenerator 1508. Each of 512 weighted signals corresponding to the samearray element N are summed in one of 512 summers 1512.

Since each group of 64 outputs from of the summers 1512 will be coupledto, and transmitted by, a different one of the 8 SANs 1410, a timingmodule 1514 is provided. The timing module 1514 adjusts when the beamelement signals 1409 are sent from the beamformer to ensure that eachgroup of 64 IF beam element signals 1409 arrives at the user beamcoverage area 1801 at the appropriate time to ensure that thesuperposition of the signals 1409 results in the proper formation of theuser beam. Alternatively, the forward beam weights can be generatedtaking into account differences in lengths and characteristics of thepaths from each SAN 1410 to the satellite 1408. Accordingly, signal 2122would be coupled to the forward beamformer 1406. In some embodiments,the timing module 1514 generates the timing pilot signal 1413transmitted from the forward beamformer 1406 to each SAN 1410. In someembodiments, one timing pilot signal 1413 is generated and split into 8copies of equal amplitude, one copy sent to each SAN 1410.Alternatively, the amplitude of the copies may be a predetermined ratio.As long as the ratio between timing pilot signals 1413 is known, RF beamelement signals 1411 can be equalized to ensure that they will superposewith one another to form the desired user spot beams. In someembodiments in which the corrections to alignment are made in the timingmodule 1514 within the beamformer 1406, each SAN 1410 returns a signal2122 derived from the SAN timing correction signal 1419 to a timingcontrol input to the beamformer to allow the forward beamformer 1406 todetermine corrections to the alignment of the signals to each SAN 1410.In some embodiments, SAN timing correction signals 1419 are then used bythe timing module 1514 to adjust the timing of the beam element signals1409. In other embodiments, the SAN timing correction signal 1419 areused by the forward beam weight generator 1508 to adjust the beamweights to account for differences in the paths from the beamformer 1406through each of the SANs 1410 to the satellite 1408. As noted above,corrections to the alignment can alternatively be made in each SAN 1410.

Once the beam element signals 1409 have been properly weighted and anynecessary timing adjustments made, each of the 512 signals 1409 arecoupled to one of the SANs 1410. That is, each of the 8 SANs 1410receives 64 beam element signals 1409 (i.e., 512/8) from the forwardbeamformer 1406. An optical transmitter 1401 within each SAN 1410receives, multiplexes and modulates those 64 beam element signals 1409that it receives onto an optical carrier.

FIG. 17 is an illustration of an optical transmitter 1401 used in someembodiments of the system 1400. The optical transmitter 1401 is similarto the optical transmitter 607 discussed above with respect to FIG. 10.However, the input signals 1409 differ, since they are beam weighted bythe beamformer 1406. Furthermore, the timing pilot signal 1413 providedby the beamformer 1406 is coupled to an optical modulator 611 andmodulated onto an optical carrier within the same band as the band ofother optical modulators 611 within the same optical band module 1403,as determined by the wavelength of the light source 654 within thatoptical modulator 608. In some embodiments, each optical band module1403 is identical. However, modulating the timing pilot signal 1413 needonly be done in one such optical band module 1403. Alternatively, asshown in FIG. 17, only one optical band module 1403 is configured tomodulate a timing pilot signal 1413. The other optical band modules 608may be similar to the optical band module 608 show in FIG. 6 anddescribed above. In either embodiment, in a system in which 8 SANs 1410each receive 64 beam element signals 1409 and modulate them onto 16optical channels within 4 different optical bands, as shown in FIG. 5,there are four optical band modules within the optical transmitter 1401in each SAN 1410.

The timing pilot signal 1413 follows the same path to the satellite asthe IF beam element signals 1409. Therefore, by comparing the arrivaltime of the timing pilot signals sent from each SAN 1410 at thesatellite 1408, differences in the arrival times of the IF beam elementsignals can be determined and correction signals can be generated andtransmitted to each SAN 1410. Similar to the optical transmitter 607,the optical channels 915 output by each optical modulator 611 shown inFIG. 17 are combined in an optical combiner 609. The composite opticalsignal 1624 is emitted from an optical lens 2002 within the opticaltransmitter 1401. The optical lens 2002 operates as an optical signaltransmitter capable of transmitting an optical signal to the satellite1408.

A composite optical signal 1624 from each of the SANs 1410 with the 64beam element signals 1409 and the timing pilot signal 1413 istransmitted to the satellite 1408 on the optical forward uplink 1402 andreceived by one of 8 optical receivers 1412 within the satellite 1408.Each of the 8 optical receivers 1412 within the satellite 1408demultiplexes the 64 optical channels 915 from the composite opticalsignal 1624.

FIG. 18 shows the components of a satellite 1408 (see FIG. 15) ingreater detail. The Satellite 1408 receives and transmits the forwardlink in accordance with some embodiments of a system using ground-basedbeamforming, as noted above with reference to FIG. 15. The components ofthe forward link of the satellite 1408 include 8 optical receivers 1412,8 amplifier/converter modules 1414 and a 512-element antenna array 1416.In some embodiments of the system 1400, similar to the embodiments shownin FIGS. 9, 13 and 16, in which there are 8 SANs (i.e., M=8), thereceived composite signal 1624 includes 64 optical channels divided into4 bands of 16 each, each of which carries a 3.5 GHz wide IF channel.Furthermore, there are K=512 user beam coverage areas 1801 and N=512elements in the antenna array. As noted elsewhere in the presentdiscussion, these numbers are provided merely as an example and for easeof discussion.

Each optical receiver 1412 is associated with a correspondingamplifier/converter module 1414. The optical receivers 1412 each includea lens module 1701, and a plurality of optical detectors, such asphotodiodes 1703. The lens module 1701 includes a lens 1702 (which insome embodiments may be similar to the lens 610 described above withrespect to FIG. 4), an optical demultiplexer 1704, a plurality ofoptical demultiplexers 1706 and a plurality of output lenses 1708.

In operation, the composite optical signal 1624 is received from each ofthe 8 SANs 1410. A lens 1702 is provided to receive each compositeoptical signal 1624. In some embodiments, the lenses 1702 can be focused(in some embodiments, mechanically pointed) at a SAN 1410 from which thelens 1702 is to receive an composite optical signal 1624. The lens 1702can later be refocused to point to a different SAN 1410. Because thelenses 1702 can be focused to receive composite optical signal 1624 fromone of several SANs 1410, the satellite 1408 can receive signals from 8SANs 1410 selected from among a larger number 8+X SANs 1410. In someembodiments X=24. Therefore, 32 different SANs 1410 are capable ofreceiving information intended to be communicated to user beam coverageareas 1801 in the system. However, only eight of the 32 SANs 1410 areselected to have information that is transmitted be received by thesatellite 1408.

The signal path of one of the composite optical signals 1624 through theforward link of the satellite 1408 is now described in detail. It shouldbe understood that each of the 8 signal paths taken by the 8 receivedcomposite optical signals 1624 through the forward link of the satellite1408 operate identically. The composite optical signal 1624 that isreceived by the lens 1702 is directed to an optical demultiplexer 1704.In a system using the modulation scheme illustrated in FIG. 9, theoptical demultiplexer 1702 splits the composite optical signal 1624 intothe four bands 907, 909, 911, 913 (see FIG. 9). That is, the opticaldemultiplexer 1704 splits the composite optical signal 1624 into thefour optical wave lengths onto which the beam element signals 1407 weremodulated by the SAN 1410 that sent the composite optical signal 1624.Each of the optical outputs from the optical demultiplexer 1704 iscoupled to a corresponding optical demultiplexer 1706. Each of the fouroptical demultiplexers 1706 output 512/(4×8) optical signals for a totalof 4×(512/(4×8)=512/8=64 optical signals. Each of the 16 optical signalsoutput from the four optical demultiplexers 1706 is directed to anoutput lens 1708. Each of the output lenses 1708 focus the correspondingoptical signal onto a corresponding photo detector, such as a photodiode1703. Each photodiode 1703 detects the amplitude envelope of the opticalsignal at its input and outputs an RF transmit beam element signal 1418corresponding to the detected amplitude envelope. Accordingly, the RFtransmit beam element signals 1418 output from the optical receivers1412 are essentially the beam element signals 1409 that were modulatedonto the optical signals by the SANs 1410.

The RF output signals are then coupled to the amplifier/converter module1414. The amplifier/converter module 1414 includes 512/8 signal paths.In some embodiments, each signal path includes a Low noise amplifier(LNA) 1710, frequency converter 1712 and PA 1714. In other embodiments,the signal path includes only the frequency converter 1712 and the PA1714. In yet other embodiments, the signal path includes only the PA1714 (the frequency converter 1712 can be omitted if the feed signalsproduced by the SANs are already at the desired forward downlinkfrequency). The frequency converter 1712 frequency converts the RFtransmit beam element signals 1418 to the forward downlink carrierfrequency. In some embodiments, the output of each upconverter 1712 isan RF carrier at a center frequency of 20 GHz. Each of the 512 outputsfrom the 8 amplifier/converter modules 1414 is coupled to acorresponding one of the 512 elements of the 512-element antenna array1416. Therefore, the antenna array 1416 transmits the 512 forwarddownlink beam element signals 1718.

FIG. 19 is an illustration of user beam coverage areas 1801 formed overthe continental United States in accordance with some embodiments. Inother embodiments, the user beam coverage areas may be located indifferent locations and with different spacing and patterns. In someembodiments, such as the embodiments shown in FIGS. 4, 8 and 12, eachfeed of an antenna is focused to direct a user spot beam to one userbeam coverage area. In other embodiments, such as shown in FIGS. 10, 11,12, 14 and, the 512 forward downlink beam element signals 1718 aresuperposed on one another to form user beams directed to user beamcoverage areas 1801. As shown in FIG. 19, user beam coverage areas aredistributed over a satellite service area that is substantially largerthan the user beam coverage areas 1801. The 512 element antenna array1416 transmits the RF beam element signals 1411 over the forwarddownlink 1404 to each of the 512 user beam coverage areas 1801. Userterminals 806 within each user beam coverage area 1801 receive the userbeam directed to that particular user beam coverage area 1801 by virtueof the superposition of the RF beam element signals 1411 transmittedfrom each of the 512 elements of the 512 element antenna array 1416.

In addition to the IF beam element signals 1418 output from each opticalreceiver 1412, each optical receiver 1412 demultiplexes a satellitetiming signal 1415 from the composite optical signal 1624. A satellitetiming signal 1415 is output from each receiver 1412 and coupled thecorresponding amp/converter module 1414. An LNA 1710 within theamp/converter module 1414 amplifies the satellite timing signal 1415.The output 1416 of the LNA 1710 is coupled to a satellite timing module1417. In some embodiments, the satellite timing module 1417 compares thesatellite timing signal 1415 received by each optical receiver 1412 todetermine whether they are aligned. The satellite timing module 1417outputs 8 SAN timing correction signals 1419, one to be returned to eachof the 8 SANs 1410. In some embodiments, each SAN timing correctionsignal 1419 is coupled to an input to a return amp/converter module 1904(see FIG. 24). Each SAN timing correction signal 1419 is amplified,frequency converted to the forward downlink frequency and coupled to aninput to one of 8 optical transmitters 1401 within the satellite 1408,similar to the optical transmitter 1401 provided in the SAN 1410. Insome embodiments, one of the eight is a reference for the other seven.Accordingly, no correction is made to the timing of the signalstransmitted from the SAN 1410 from which the reference satellite timingsignal was sent. Therefore, no SAN timing correction signal 1419 is sentfor that SAN 1410. The SAN timing correction signal 1419 is modulatedonto each composite optical signal transmitted by the satellite 1408 toeach SAN 1410.

Each SAN timing correction signal 1419 provides timing alignmentinformation indicating how far out of alignment the timing pilot signal1413 is with respect to the other timing pilot signals (e.g., thereference satellite timing signal 1415). In some embodiments, the timinginformation is transmitted through the SANs 1410 to a timing module 1514(see FIG. 16) in the beamformer 1406. The timing module 1514 adjusts thealignment of the beam elements prior to sending them to each SAN 1410.Alternatively, the timing alignment information is used by each SAN 1410to adjust the timing of the transmissions from the SAN 1410 to ensurethat the RF beam element signals 1411 from each SAN 1410 arrive at thesatellite 1408 in alignment. FIG. 20 is an illustration of an opticaltransmitter 1460 having a timing module 1462 for adjusting the timing ofthe beam element signals 1409 and the timing pilot signal 1413. Thetiming module 1462 receives a timing correction signal 1464 fromsatellite 1408 over the return downlink (discussed in further below).The timing module applies an appropriate delay to the signals 1409, 1413to bring the signals transmitted by the SAN 1410 into alignment with thesignals transmitted by the other SANs 1410 of the system 1400.

In an alternative embodiment, timing adjustments can be made to the RFbeam element signals 1411 within the satellite based on control signalsgenerated by the satellite timing module 1417. In some such embodiments,the control signals control programmable delays placed in the signalpath between the optical receiver 1412 and the antenna array 1416 foreach RF beam element signal 1411.

In an alternative embodiment, at least two of the satellite timingsignals 1415 are transmitted from the satellite back to each SAN 1410.The first is a common satellite timing signal 1415 that is transmittedback to all of the SANs. That is, one of the received satellite timingsignals 1415 is selected as the standard to which all others will bealigned. The second is a loop back of the satellite timing signal 1415.By comparing the common satellite timing signal 1415 with the loop backsatellite timing signal 1415, each SAN 1410 can determine the amount ofadjustment needed to align the two signals and thus to align the IF beamelement signals 1418 from each SAN 1410 within the satellite 1410.

FIG. 21 is a system 1450 in which each of the K forward beam inputsignals 1452 contain S 500 MHz wide sub-channels. In some embodiments,K=512 and S=7. For example, in some embodiments, seven 500 MHz widesub-channels are transmitted to one user coverage area 1801. FIG. 22 isan illustration of a beamformer 1300 in which forward beam input signals1452 comprise seven 500 MHz wide sub-channels, each coupled to a uniqueinput to the beamformer 1300. Accordingly, as noted above, thesub-channels can be beamformed after being combined into an IF carrier,as shown in FIGS. 14, 15. Alternatively, as shown in FIG. 13, thesub-channels 1452 can be beamformed before being combined using thebeamformer 1300. Accordingly, the beamformer 1300 outputs S×N beamelement signals, with (S×N)/M such beam element signals being sent toeach SAN 1410. In the example system 1450, S=7, N=512 and M=8. As notedabove, these numbers are provided as a convenient example and are notintended to limit the systems, such as the system 1450, to theseparticular values.

FIG. 22 is a simplified block diagram of a beamformer 1300 in which eachcarrier comprises S sub-channels 1452, where S=7. Each of thesub-channels 1452 is provided as independent input to a matrixmultiplier 1301 within the beamformer 1300. Therefore, 512×7sub-channels 1452 are input to the matrix multiplier 1301, where thereare 512 user spot beams to be formed and 7 is the number of sub-channelsin each carrier; that is, K=512 and S=7. The 512-way splitter 1304receives each of the 512×7 sub-channels 1407, where 512 is the number ofelements in the antenna array 1416. Alternatively, N may be any numberof antenna elements. Each sub-channel 1452 is split 512 ways.Accordingly, 512×512×7 signals are output from the splitter 1304 in athree-dimensional matrix. The signals 1, 1, 1 through 1, K, 1 (i.e., 1,512, 1 where K=512) are weighted and summed in a weighting and summingmodule 1306. Likewise, the signals 1, 1, 7 through 1, 512, 7 areweighted and summed in a weighting and summing module 1313. In similarfashion, each of other weighting and summing modules weight receiveoutputs from the splitter 1304, and weight and sum the outputs. The512×7 outputs from the weighting and summing modules 1306, 1313 arecoupled to the inputs of a timing module 1514. The timing modulefunctions essentially the same as the timing module 1514 of thebeamformer 1406 discussed above. The beamformer 1300 outputs 512×7 beamelement signals 1454 to the SANs 1410. Each of the 8 SANs 1410 comprisesan IF combiner 1602.

FIG. 23 is an illustration of a SAN 1456 of system 1450. In someembodiments, a first baseband to IF converter 805 operates in similarfashion to the baseband to IF converter 805 discussed above with respectto FIG. 10. The converter 805 outputs a signal 811 that is a combinationof seven 500 MHz beam element signals 1454. In addition, in someembodiments, at least one of the baseband to IF converters 1605 includesan additional frequency converter 1607. The additional frequencyconverter 1607 receives the timing pilot signal 1413 from the beamformer1300. The timing pilot signal 1413 is combined with the beam elementsub-channels 1452 and coupled to the optical transmitter 607. Each ofthe IF signals 811 coupled to the optical transmitter 607 are combinedin the optical combiners 609 of each SAN 1410 to form the transmittedcomposite optical signal 1624. The timing pilot signal 1413 is coupledto the input of a frequency converter 1607. The frequency converter 1607places the timing pilot signal at a frequency that allows it to besummed with the beam element signals 1454 by the summer 1608.Alternatively, the timing pilot signal 1413 can be directly coupled toan additional optical modulator 1610 dedicated to modulating the timingpilot signal 1413. The output of the additional modulator 1610 iscoupled to the combiner 609 and combined with the other signals on aunique optical channel dedicated to the timing pilot signal.

FIG. 24 is an illustration of a return link for the system 1400 havingground-based beamforming. User terminals 806 located within a pluralityof 512 user beam coverage areas 1801 transmit RF signals to thesatellite 1408. An 512-element antenna array 1902 on the satellite 1408(which may or may not be the same as the antenna array 1416) receivesthe RF signals from the user terminals 806. 512/8 outputs from the512-element antenna array 1902 are coupled to each of the 8amplifier/converter modules 1904. That is, each element of the antennaarray 1902 is coupled to one LNA 1906 within one of theamplifier/converter modules 1904. The output of each LNA 1906 is coupledto the input to a frequency converter 1908 and a pre-amplifier 1910. Theamplified output of the LNA 1906 frequency down-converted from RF useruplink frequency to IF. In some embodiments, the IF signal has abandwidth of 3.5 GHz. In some embodiments, the pre-amp 1910 providesadditional gain prior to modulation onto an optical carrier. The outputsof each amplifier/converter modules 1904 are coupled to correspondinginputs to one of 8 optical transmitters 1401, similar to the opticaltransmitter 607 of FIG. 4. Each of 8 optical transmitters 1401 outputsand transmits an optical signal to a corresponding SAN 1410. The SAN1410 receives the optical signal. The SAN 1410 outputs 512/8 return beamelement signals 1914 to a downlink beamformer 1916. The downlinkbeamformer 1916 processes the return beam element signals 1914 andoutputs 512 beam signals 1918, each corresponding with one of 512 userbeam coverage areas 1801.

The IF signals provided to the optical transmitter 607 from theamplifier/converter module 1904 are each coupled to one of 512/8 opticalmodulators 611. For example, if there are 512 elements in the antennaarray 1902 (i.e., N=512) and there are 8 SANs 1410 in the system 1900,then 512/8=64. In a system in which the IF signals are modulated ontowavelengths divided into 4 bands, such as shown in FIG. 9, the opticalmodulators 611 are grouped together in optical band module 608 having512/(4×8) optical modulators 611.

Each optical modulator 611 is essentially the same as the uplink opticalmodules 611 of the SAN 1410 shown in FIG. 10, described above. Eachoptical modulator 611 within the same optical band module 608 has alight source 654 that produces an optical signal having one of 16wavelengths. Accordingly, the output of each optical modulator 611 willbe at a different wavelength. Those optical signals generated within thesame optical band module 608 will have wavelengths that are in the sameoptical band (i.e., in the case shown in FIG. 9, for example, theoptical bands are 1100 nm, 1300 nm, 1550 nm and 2100 nm). Each of thoseoptical signals will be in one of 16 optical channels within the bandbased on the wavelengths 2. The optical outputs from each opticalmodulator 611 are coupled to an optical combiner 609. The output of theoptical combiner 609 is a composite optical signal that is transmittedthrough an optical lens 2016 to one of the SANs 1410. The optical lens2016 can be directed to one of several SANs 1410. Accordingly, the 8optical transmitters each transmit one of 8 optical signals to one of 8SANs 1410. The particular set of 8 SANs can be selected from a largergroup of candidate SANs depending upon the quality of the optical linkbetween the satellite and each candidate SAN.

FIG. 25 is an illustration of one of the SANs 1410 in the return link Δnoptical receiver 622 comprises lens 2102 that receives optical signalsdirected to the SAN 1410 from the satellite by the lens 2016. An opticalband demultiplexer 2104 separates the optical signals into opticalbands. For example, in some embodiments in which there are four suchbands, each of the four optical outputs 2106 are coupled to an opticalchannel demultiplexer 2108. The optical channel demultiplexer 2108separates the 512/(4×8) signals that were combined in the satellite1408. Each of the outputs from the four optical channel demultiplexers2108 are coupled to a corresponding lens 2110 that focuses the opticaloutput of the optical channel demultiplexers 2108 onto an opticaldetector, such as a photodiode 2112. Output signals 2116 from thephotodiodes 2112 are each coupled to one of 512/8 LNAs 2114. The outputfrom each LNA 2114 is coupled to the return link beamformer 1916 (seeFIG. 24). In addition, one channel output from the optical receiver 622outputs a timing correction signal 1464 that is essentially the SANtiming correction signal 1419 (see FIG. 18) that was provided by thesatellite timing module to the return amplifier/converter module 1414.In some embodiments, the timing correction signal 1464 is coupled to atiming pilot modem 2120. The timing pilot modem outputs a signal 2122that is sent to the forward beamformer 1406. In other embodiments, thetiming correction signal 1464 is coupled to a timing control input ofthe timing module 1462 (see FIG. 20) discussed above.

FIG. 26 illustrates in greater detail, a return beamformer 1916 inaccordance with some embodiments of the disclosed techniques. Each ofthe 512 outputs signals 2116 is received by the return beamformer 1916from each of the SANs 1410. The return beamformer comprises abeamforming input module 2203, a timing module 2201, matrix multiplier2200 and a beamformer output module 2205. The matrix multiplier 2200includes a K-way splitting module 2202 and 512 weighting and summingmodules 2204. The matrix multiplier 2200 multiplies a vector of beamsignals by a weight matrix. Other arrangements, implementations orconfigurations of a matrix multiplier 2200 can be used. Each signal 2116is received by the beamformer 1916 in the beamformer input module 2203and coupled to the timing module 2201. The timing module 2201 ensuresthat any differences in the length and characteristics of the path fromthe satellite to the SAN 1410 and from the SAN 1410 to the returnbeamformer 1916 is accounted for. In some embodiments, this may be doneby transmitting one pilot signal from the return beamformer 1916 to eachSAN 1410, up to the satellite and retransmitting the pilot signal backthrough the SAN 1410 to the return beamformer 1916. Differences in thepaths between the return beamformer 1916 and the satellite can bemeasured and accounted for.

The output of the timing module is coupled to a K-way splitter 2202 thatsplits each signal into 512 identical signals. 512 unique signals areapplied to each of 512 weighting and summing circuits 2204. Each of the512 unique signals is weighted (i.e., the phase and amplitude areadjusted) within a weighting circuit 2206, such that when summed in asumming circuit 2208 with each of the 512 other weighted signals, areturn link user beam is formed at the output of the return beamformer.

Each of the architectures described above are shown for an opticaluplink to the satellite. In addition, an optical downlink from thesatellite to SANs on Earth operates essentially the reverse of theoptical uplinks described. For example, with regard to the architectureshown in FIG. 4, an optical downlink from the satellite 602 to the SAN604 provides a broadband downlink Rather than lenses 610 for receivingthe optical uplink, lasers are provided for transmitting an opticaldownlink. Furthermore, rather than the bi-phase modulator 614 generatinga BPSK modulated signal to be transmitted on an RF carrier, the bi-phasemodulator modulates the optical signal using an optical binarymodulation scheme. Similarly, an optical downlink can be provided usingan architecture similar to that shown in FIG. 4. In this embodiment, themodulator 614 would instead be a QAM demodulator that receives a QAMmodulated RF or IF signal and demodulates the bits of each symbol andusing binary optical modulation of an optical signal for transmission onthe optical downlink. In the embodiment of the architecture shown inFIG. 8, a similar architecture can be used in which the feeder downlinkfrom the satellite to the SAN is optical, the received RF signals fromthe user terminals 842, 844 are directed by a matrix switch to a laserpointed at the particular SAN selected to receive the signal. The RFsignal is RF modulated onto the optical signal similar to the way thefeeder uplink optical signal is RF modulated by the baseband/RF modem811 in the SAN 802.

In some embodiments, the lasers used to transmit an optical feederdownlink signal are pointed to one of several SANs. The SANs areselected based upon the amount of signal fade in the optical path fromthe satellite to each available SAN, similar to the manner in which theSANs of FIGS. 4, 8 and 12 are selected.

Although the disclosed techniques are described above in terms ofvarious examples of embodiments and implementations, it should beunderstood that the particular features, aspects and functionalitydescribed in one or more of the individual embodiments are not limitedin their applicability to the particular embodiment with which they aredescribed. Thus, the breadth and scope of the claimed invention shouldnot be limited by any of the examples provided in describing the abovedisclosed embodiments.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, should be construed as open ended as opposedto limiting. As examples of the foregoing: the term “including” shouldbe read as meaning “including, without limitation” or the like; the term“example” is used to provide examples of instances of the item indiscussion, not an exhaustive or limiting list thereof; the terms “a” or“an” should be read as meaning “at least one,” “one or more” or thelike; and adjectives such as “conventional,” “traditional,” “normal,”“standard,” “known” and terms of similar meaning should not be construedas limiting the item described to a given time period or to an itemavailable as of a given time, but instead should be read to encompassconventional, traditional, normal, or standard technologies that may beavailable or known now or at any time in the future. Likewise, wherethis document refers to technologies that would be apparent or known toone of ordinary skill in the art, such technologies encompass thoseapparent or known to the skilled artisan now or at any time in thefuture.

A group of items linked with the conjunction “and” should not be read asrequiring that each and every one of those items be present in thegrouping, but rather should be read as “and/or” unless expressly statedotherwise. Similarly, a group of items linked with the conjunction “or”should not be read as requiring mutual exclusivity among that group, butrather should also be read as “and/or” unless expressly statedotherwise. Furthermore, although items, elements or components of thedisclosed techniques may be described or claimed in the singular, theplural is contemplated to be within the scope thereof unless limitationto the singular is explicitly stated.

The presence of broadening words and phrases such as “one or more,” “atleast,” “but not limited to” or other like phrases in some instancesshall not be read to mean that the narrower case is intended or requiredin instances where such broadening phrases may be absent. The use of theterm “module” does not imply that the components or functionalitydescribed or claimed as part of the module are all configured in acommon package. Indeed, any or all of the various components of amodule, whether control logic or other components, can be combined in asingle package or separately maintained and can further be distributedin multiple groupings or packages or across multiple locations.

Additionally, the various embodiments set forth herein are describedwith the aid of block diagrams, flow charts and other illustrations. Aswill become apparent to one of ordinary skill in the art after readingthis document, the illustrated embodiments and their variousalternatives can be implemented without confinement to the illustratedexamples. For example, block diagrams and their accompanying descriptionshould not be construed as mandating a particular architecture orconfiguration.

What is claimed is:
 1. A satellite communication system comprising: abeamformer comprising: a plurality of beamformer inputs, each configuredto receive signals to be directed to a user spot beam; a plurality ofbeamformer outputs, each configured to output a beam element signal; apilot output configured to output a timing pilot signal; and a SANcomprising: a plurality of optical modulators each having an electricalinput and an optical output, at least one electrical input coupled to acorresponding output from the beamformer and at least one electricalinput configured to receive the timing pilot signal; an optical combinercoupled to the optical outputs of the plurality of optical modulators;and an optical lens coupled to the output of the optical combiner andconfigured to transmit an optical signal; a satellite comprising: asatellite optical receiver having a steerable lens pointed at the SANand configured to receive the optical signal, the satellite opticalreceiver having a plurality of radio frequency (RF) outputs; a pluralityof power amplifiers (PA), each having an input and an output, the inputscoupled to the RF outputs; and an antenna array having a plurality ofantenna elements, each antenna element having an input coupled to acorresponding one of the PA outputs.
 2. The system of claim 1, whereinthe steerable lens can be positioned by rotation about at least two axisin response to ground commands.
 3. The system of claim 1, wherein thesteerable lens can be positioned by rotation about at least two axis inresponse to on-board processing.
 4. The system of claim 1, furthercomprising a second SAN comprising: a plurality of second opticalmodulators each having an electrical input and an optical output, atleast one electrical input coupled to a corresponding output from thebeamformer and at least one electrical input configured to receive thetiming pilot signal; a second optical combiner coupled to the opticaloutputs of the plurality of second optical modulators; and a secondoptical lens coupled to the output of the second optical combiner andconfigured to transmit a second optical signal; wherein the satellitefurther comprises: a second satellite optical receiver having asteerable lens pointed at the second SAN and configured to receive thesecond optical signal, the satellite optical receiver having a pluralityof second radio frequency (RF) outputs; a plurality of second poweramplifiers (PA), each having an input and an output, the inputs coupledto the second RF outputs; and wherein the antenna array has a pluralityof second antenna elements, each second antenna element having an inputcoupled to a corresponding one of the second PA outputs.
 5. The systemof claim 4, wherein the SANs each have a SAN optical receiver configuredto receive an optical signal from a satellite and to demultiplex anddemodulate a timing correction signal from the optical signal.
 6. Thesystem of claim 5, wherein the beamformer further comprises a timingmodule having a timing control input, and wherein each SAN furthercomprises a timing pilot modem coupled to the SAN optical receiver, thetiming pilot modem having an output coupled to the beamformer timingcontrol input, wherein the timing module within the beamformer generatesa delay in the transmission of beam elements signals in response tosignals presented to the timing control input.
 7. The system of claim 5,where the SANs each further comprise a timing module coupled between thebeamformer and each optical modulator, the timing module having a timingcontrol input coupled to receive the timing correction signal from theSAN optical receiver, wherein the timing module within the SAN generatesa delay in the transmission of beam elements signals in response tosignals presented to the timing control input.
 8. The system of claim 1,wherein the antenna patterns of at least some of the antenna elementsoverlap such that signals transmitted therefrom will be superposed uponone another and thus coherently combine to form a user spot beam.
 9. Thesystem of claim 1, wherein the satellite optical receiver comprises anoptical demultiplexer having an input and a plurality of outputs, eachoutput associated with a corresponding optical wavelength, wherein thewavelengths associated with the outputs of the optical demultiplexer aregrouped in optical bands and wherein wavelengths in the same opticalband define unique optical channels.
 10. The system of claim 1, whereinthe satellite optical receiver comprises a plurality of satellitereceiver optical detectors each having an RF output coupled to acorresponding RF output of the satellite optical receiver.
 11. Thesystem of claim 10, wherein the RF signal output from the satellitereceiver detector has an amplitude that tracks the intensity of theoptical signal applied to the satellite receiver optical detector input.12. The system of claim 11, wherein the satellite receiver opticaldetector is a photo diode.
 13. The system of claim 1, further comprisingat least one frequency converter coupled between a corresponding one ofthe optical receiver RF outputs and a corresponding one of the antennaelement inputs.
 14. The system of claim 1, wherein each of the pluralityof optical modulators comprise at least one light source operating at anoptical wavelength within one of four optical bands, wherein the fouroptical bands are centered at approximately 1100 nm, 1300 nm, 1550 nmand 2100 nm, wherein the wavelength of each light source within the sameoptical band is spaced apart by approximately 100 GHz, and wherein theelectrical input to the plurality of optical modulators is furtherconfigured to receive a signal having a bandwidth of approximately 3.5GHz.
 15. The system of claim 1, the SAN further comprising a pluralityof baseband to IF converters, each having: an input coupled acorresponding one of the plurality of beamformer outputs, and an outputcoupled to the electrical input of a corresponding one of the pluralityof optical modulators.
 16. The system of claim 1, the SAN furthercomprising a SAN optical receiver configured to receive an opticalsignal and to output a plurality of beam element signals.
 17. The systemof claim 16, wherein the SAN optical receiver is further configured tooutput at least one timing correction signal.